Guest Editorials

The Road to Improved Long-Term Pavement Performance

The public road system in this country consists of 3.9 million highway miles and is an expensive network for Federal, State, and local governments to build, maintain, and manage. Materials costs tend to increase along with the price of oil; therefore, departments of transportation can hardly afford to over- or underbuild. Each project needs to be done right the first time, and the facility needs to last.

Understanding why some pavements perform better than others is the key to building and maintaining a cost-effective highway system. The answers, however, are not simple. Engineers often are asked, "Why don't we build roads like the Romans did? Their roads have stood the test of time." Truthfully, those roads would hardly serve the needs of today's multi-ton, high-speed "chariots." The emphasis today is on speed, safety, and long-term performance.

Engineers have been working to understand pavement performance for the past century, and the research continues. In the 1950s, the American Association of State Highway Officials (AASHO), predecessor of the American Association of State Highway and Transportation Officials, and the Federal Government conducted a large-scale study to better define the performance of roads under heavy loads. That research came to be known as the AASHO Road Test. The resulting design procedures have been the basis for explaining pavement performance for the past 50 years. Unfortunately, during that time, truck tire pressures and loads have increased significantly. Engineers using these design procedures have had to extrapolate far beyond the design loads experienced in the AASHO study. Further, the researchers could not reliably document environmental damage in the 1950s experiment, as the accelerated loading test lasted only a few years.

In 1987, to address these information gaps, the Federal Highway Administration (FHWA) and its partners initiated a comprehensive, long-term study of inservice pavements. The Long-Term Pavement Performance (LTPP) research includes a series of rigorous field experiments monitoring more than 2,500 asphalt and portland cement concrete pavement test sections across the United States and Canada. (For more detail about the program, see "LTPP Keeps Rolling" in this issue of Public Roads.)

The LTPP data made possible development of a new pavement design guide, the Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures, which replaces older design procedures that were based on lighter truck loads and lacked the benefit of information on the effects of environmental damage.

So what's next? After the program's first 20-year evaluation period, the focus has shifted to preserving the vast quantities of collected data and evaluating those data to improve the industry's knowledge of pavement behavior. FHWA recently added a 60-terabyte server to house the LTPP data. Another major focus is development of new user-friendly methods for obtaining and organizing data for evaluation. The goal is to empower researchers, scientists, and engineers representing governments and universities across the globe in their quest for answers to questions about pavement performance. And if you need help, the LTPP team stands ready to assist.